Investigating the role of nuclear receptor proliferator-activated receptor delta (PPARδ) in aging and metabolic models of osteoarthritis

Abstract

Purpose: Osteoarthritis (OA) is a complex disease that can be differentiated based on the risk factors and pathological mechanisms that drive joint damage. Upon proteomic analysis of human osteoarthritic cartilage tissue, several proteins that are linked to lipid catabolism have been identified, including peroxisome proliferator-activated receptor delta (PPARδ), a class II nuclear receptor that is activated by naturally occurring fatty acids (Wu et al., 2007). PPARδ can be activated by retinoic acid, a vitamin A metabolite, and has been shown to play a role in multiple cellular processes including fatty acid oxidation, inflammation, cell proliferation, and apoptosis. Previous in vitro studies from our lab have shown increased expression of Mmp2/3 and Adamts 4/5 genes in chondrocytes treated with the PPARδ agonist (Ratneswaran, 2017). Similarly, in a post-traumatic model of OA, cartilage-specific Pparδ KO mice were significantly protected from OA progression compared to controls after undergoing DMM surgery. This data suggests that inhibition of PPARδ activity can protect against the development of post-traumatic OA in mouse models. However, the role of PPARδ in other models of OA have not yet been studied. Given its action in a post-traumatic of OA and its role in regulating metabolism, we hypothesize that inhibiting PPARδ will protect from OA during aging and obesity. Methods: Male mice displaying cartilage-specific PPARδ gene deletion were generated by breeding Pparδfl/fl mice with mice that express Cre-Recombinase gene under the control of the collagen 2 promoter. In order to investigate OA in an aging model, male and female C57BL/6 mice were raised to 6, 12, 20, and 24 months of age. Prior to sacrifice, the CatWalk XT system was used to measure gait and locomotion. For histological analysis, synovial joints were harvested, and classical measures of histopathology such as Toluidine Blue staining, immunohistochemistry staining, and histological scoring according to the Osteoarthritis Research Society International system (OARSI) were used to evaluate the structural progression of OA in these mouse models. In order to study a metabolic model of OA, both PPARδ knockout mice and control littermates (N=10) at 10 weeks of age are fed a 45% high-fat high-carbohydrate western (TD.10885) diet (WD) or a control diet (CD) for 12, 24 or 40 weeks before being sacrificed for subsequent analysis. Spontaneous activity within cages will be used as a surrogate measure of joint pain. Analyses will be performed as above. Results: PPARδ-KO mice exhibited significantly increased articular cartilage thickness in the ankle joint compared to litter-matched controls during aging, suggesting PPARδ may also play a protective role in age-associated OA. However, this effect was not observed in all joints tested, suggesting a joint-specific role of PPARδ. The knees, elbows, ankles and temporomandibular joints were isolated from wild-type C57BL/6 male mice as a preliminary study to determine whether western diet (WD) induces OA. Although WD mice appeared healthy with no differences in OARSI scores compared to controls at both the 12- and 24- week time point, mice on WD at the 40-week time point appear to show more cartilage damage and subchondral bone remodeling (N=3). We are in the process of analyzing the progression of osteoarthritic symptoms including articular cartilage damage using the OARSI scoring system, subchondral bone remodeling, osteophyte formation, and meniscal damage within the medial and lateral surfaces of the knee joint, and are running trials using Pparδ KO mice on these diets. Conclusions: WD appears to induce obesity-induced OA in mice enrolled on the diet for over 40 weeks, however a longer time point may be required to induce more significant OA symptoms. This study suggests that PPARδ may have a protective role not only in an injury model, but also in age-associated, at least in some joints, and possibly in obesity-associated OA.